Abiotic stress is the primary cause of crop loss worldwide, causing average yield losses of more than 50% for major crops (Boyer, J. S. (1982) Science 218:443-448; Bray, E. A. et al. (2000) In Biochemistry and Molecular Biology of Plants, Edited by Buchannan, B. B. et al., Amer. Soc. Plant Biol., pp. 1158-1249). Among the various abiotic stresses, drought is a major factor that limits crop yield worldwide. Exposure of plants to a water-limiting environment during various developmental stages appears to activate various physiological and developmental changes. Understanding of the basic biochemical and molecular mechanism for drought stress perception, transduction and tolerance is a major challenge in biology.
Photosynthetic reactions in higher plants depend on chloroplast thylakoid membrane system. Chloroplast thylakoid assembly and maintenance require a continuous supply of membrane constituents. Galactose-containing glycerolipids are predominant lipid components of photosynthetic membranes in plants, algae, and cyanobacteria. The two most common galactolipids are mono- and digalactosyldiacylglycerol (MGDG and DGDG), which account for about 50 and 25 mol % of total thylakoid lipids, respectively. About 80% of all plant lipids are associated with photosynthetic membranes, and MGDG is considered to be the most abundant membrane lipid on earth. Galactolipids play an important role in the organization of photosynthetic membranes and in their photosynthetic activities.
In plants, MGDG is synthesized in two unique steps: (i) the conversion of UDP-D-glucose (UDP-Glc) into UDP-D-galactose (UDP-Gal) by an UDP-glucose 4-epimerase (UGE), and (ii) the transfer of a galactosyl residue from UDP-Gal to diacylglycerol (DAG) for synthesis of the final product by MGDG synthase (MGD1). MGD1 is localized in the inner chloroplast envelope membrane and uses UDP-Gal as a substrate.
Plants possess a sophisticated sugar biosynthetic machinery comprising families of nucleotide sugars that can be modified at their glycosyl moieties by nucleotide sugar interconversion enzymes to generate different sugars. UDP-glucose 4-epimerase (also UDP-galactose 4-epimerase, UGE; EC 5.1.3.2) catalyzes the interconversion of UDP-Glc and UDP-Gal. UGEs identified from plants lack transmembrane motifs and signal peptides and appear to exist as soluble entities in the cytoplasm. Generally, plant UDP-Glc epimerase enzymes are localized to the cytosol, where their substrates UDP-Glc and UDP-Gal are present at high levels. As a precursor for the synthesis of the galactolipid MGDG in chloroplasts, UDP-Gal is generally thought to be mobilized from the cytosol, because the UDP-Gal concentration is relatively low within plastids and MGDG synthase (MGD1) is associated with the inner envelope membrane
To gain insight into genes controlling photosynthetic activity and carbon assimilation in plants, a rice stunted growth mutant (phd1) with decreased photoassimilate and yield production was identified. A novel chloroplast-localized UDP-Glc epimerase involved in UDP-Gal supply for chloroplast galactolipid biosynthesis during photosynthetic membrane biogenesis is disclosed herein.